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    SubjectRe: [PATCH] ARM: DMA: Fix kzalloc flags in __iommu_alloc_buffer()
    On Mon, Dec 8, 2014 at 7:24 PM, Arnd Bergmann <arnd@arndb.de> wrote:
    > On Monday 08 December 2014 17:39:27 Alexandre Courbot wrote:
    >> There doesn't seem to be any valid reason to allocate the pages array
    >> with the same flags as the buffer itself. Doing so can eventually lead
    >> to the following safeguard in mm/slab.c to be hit:
    >>
    >> BUG_ON(flags & GFP_SLAB_BUG_MASK);
    >>
    >> This happens when buffers are allocated with __GFP_DMA32 or
    >> __GFP_HIGHMEM.
    >>
    >> Fix this by allocating the pages array with GFP_KERNEL to follow what is
    >> done elsewhere in this file. Using GFP_KERNEL in __iommu_alloc_buffer()
    >> is safe because atomic allocations are handled by __iommu_alloc_atomic().
    >>
    >
    > I think you need to carry over the GFP_ATOMIC flag if that is set by the
    > caller, but not the GFP_HIGHMEM or GFP_DMA32. Not sure if it's better
    > to mask out flags from the caller mask, or to start with GFP_KERNEL
    > and adding in extra bits.

    I thought the issue of atomicity is already handled by
    __iommu_alloc_buffer's caller (arm_iommu_alloc_attrs):

    if (!(gfp & __GFP_WAIT))
    return __iommu_alloc_atomic(dev, size, handle);
    ....
    pages = __iommu_alloc_buffer(dev, size, gfp, attrs);

    Isn't the interesting property about GFP_ATOMIC that it does not
    include __GFP_WAIT? I may very well misunderstand the issue, sorry if
    that's the case.


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